1. Field of the Invention
The present invention relates to a method of separating isomers of nitrotoluic acid. More particularly, the present invention relates to a method of separating 3-nitro-o-toluic acid and 5-nitro-o-toluic acid into two individual components at high yields by producing salts by way of adding an aromatic organic base to a mixture of 3-nitro-o-toluic acid and 5-nitro-o-toluic acid and efficiently recovering both 3-nitro-o-toluic acid and 5-nitro-o-toluic acid by taking advantage of the difference between said salts in their solubility to water or a mixture of water and a water-soluble organic compound.
2. Descriptions of the Related Art
3-Nitro-o-toluic acid and 5-nitro-o-toluic acid are highly important compounds as raw materials for pharmaceuticals, agrochemicals, dyestuff, etc.
There has been known the process of nitrating o-toluic acid for manufacturing 3-nitro-o-toluic acid and 5-nitro-o-toluic acid. In such a nitration process, the reaction gives, as its product, a mixture of 3-nitro-o-toluic acid and 5-nitro-o-toluic acid, which need to be separated from each other.
3-Nitro-o-toluic acid and 5-nitro-o-toluic acid both have high melting points and high boiling points, hence it is rendered quite difficult to separate them by distillation or the like. As a method of separating the isomers from each other, a troublesome method like the fractional recrystallization method which is described in Japanese Patent Publication No. 1975-7592 is necessitated. Even if the recrystallization method is employed, it is still difficult to accomplish the separation in a single step.
Besides, it is possible to change the production ratio between 3-nitro-o-toluic acid and 5-nitro-o-toluic acid by altering conditions of the nitration reaction. For instance, when severe reaction conditions are selected, the ratio of the amount of 5-nitro-o-toluic acid produced to the amount of 3-nitro-o-toluic acid produced is improved, and by recrystallizing the nitration product thus obtained, there can be obtained 5-nitro-o-toluic acid having a high purity. In the case where mild reaction conditions are selected, however, the amount of 3-nitro-o-toluic acid produced never exceeds the amount of 5-nitro-o-toluic acid produced, and hence it is impossible to produce 3-nitro-o-toluic acid at a high yield.
Another generally conceivable method of producing 3-nitro-o-toluic acid and 5-nitro-o-toluic acid is the separation by adjusting the pH in the precipitation process using an acid. It is, however, impossible to produce both 3-nitro-o-toluic acid and 5-nitro-o-toluic acid in satisfactory yields and qualities.
For instance, even though it is possible to produce 5-nitro-o-toluic acid at a high yield by the precipitation process (which is performed by, after dissolving a metallic salt of nitrotoluic acid into a solution, adjusting the pH value of the solution by adding an acid), 3-nitro-o-toluic acid cannot be produced at a high yield by recovering crystals from the mother liquor from which 5-nitro-o-toluic acid has been already separated since 5-nitro-o-toluic acid is deposited in the first place.
The inventors of the present application, after having studied methods of overcoming the aforementioned difficulties and separating from a mixture of 3-nitro-o-toluic acid and 5-nitro-o-toluic acid both of these isomers easily at high yields, discovered that the salts produced from the isomers by adding an aromatic organic base to nitro-o-toluic acid exhibit their inherent solubilities to water or a mixed solvent consisting of water and a water-soluble organic compound. The inventors observed that the solubilities of 3-nitro-o-toluic acid and 5-nitro-o-toluic acid differed from each other to a great extent, and discovered that both 3-nitro-o-toluic acid and 5-nitro-o-toluic acid can be recovered at high yields easily and efficiently by separating such a solution into solids and a liquid. The inventors have thus completed the present invention.